The present invention concerns a method involving an adjustable-crown roll (a variable-crown roll), in which a series of hydraulic loading elements is used and which is placed in a plane of the nip that is formed between the variable-crown roll and a counter-roll, so as to act between a central axle of the variable-crown roll and a cylindrical mantle thereof. The cylindrical mantle is fitted around the axle so that the mantle can revolve about the axle, which is supported by axle journals thereof through deflection bearings or equivalent supporting structures. The central axle is shaped, regarding the cross-section thereof, such that the axle has sufficiently high rigidity in the nip plane.
The present invention further concerns a roll device intended for carrying out the method of the present invention, this roll device comprising a massive central axle and a roll mantle arranged to revolve about the axle, whereby, in a space between the central axle and the roll mantle, a series of glide-shoe loading-piston elements or equivalent is fitted, by means of which the profile of the nip that is formed by the variable-crown roll with a counter-roll thereof can be adjusted. The glide-shoe-piston series or equivalent can be fitted so as to act substantially in the plane of the nip. The stationary axle is supported from outside the roll mantle by means of articulation bearings or equivalent supports.
In paper machines, such rolls are commonly used to form a dewatering press nip, a smoothing nip, or a calendering nip with a counter-roll. In these purposes of use, it is important for the distribution of the linear load in the nip,.i.e. the profile in the axial direction of the rolls, to be unvarying or adjustable as desired e.g., in order to control the moisture profile and/or the thickness profile (caliper) of the web in a transverse direction. For this purpose, various variable-crown or adjustable-crown rolls are known in the prior art, by means of which attempts have been made to act upon the distribution of linear load in a nip (the term "variable-crown roll" will be used below, to denote all types of variable-crown or adjustable-crown rolls).
Several different variable-crown rolls for paper machines are known in the prior art. As a rule, these rolls comprise a massive or tubular, stationary roll axle and a roll mantle arranged to revolve about the axle. Arrangements of glide shoes and/or a chamber or a series of chambers for pressure fluid are fitted between the axle and the mantle to act upon an inner face of the mantle, so that axial profile of the mantle at the nip can be aligned or adjusted as desired. As a rule, the nips formed by such rolls, such as press nips or calendering nips, are loaded by means of loading forces applied to the axle journals of the variable-crown roll and of the counter-roll.
So-called variable-crown rolls without loading arms are known in the prior art, in which the stroke length of the piston-cylinder elements of the hydraulic loading elements has been arranged to be large enough so that it is also sufficient for producing the opening movement and correspondingly the closing movement of the nip, and even so that loading arms acting upon ends of the roll axle and provided with power units are not needed at all. In such variable-crown rolls without loading arms, the hydraulic loading elements which act upon an inner face of the roll mantle, also produce a nip loading pressure per se, in addition to crown variation and nip profile.
When variable-crown rolls are used as press rolls, it is necessary to open the nips formed by the same, e.g., for servicing operations such as replacement of press felts. When a variable-crown roll is used as a calendar roll, the calendering nips must be opened, e.g., for the threading of the web.
Drawbacks of the prior art variable-crown rolls without loading arms include complicated construction and difficulties in making the stroke length of the piston-cylinder elements between the stationary central axle and the roll mantle sufficiently long, and in producing sufficiently high loading and crown-variation forces by means of the piston-cylinder elements.
With respect to the prior art most closely related to the present invention, reference is made to Finnish Pat. No. 57,004 (corresponding to U.S. Pat. No. 3,885,283) and to Finnish Pat. No. 67,923 (corresponding to DE-OS No. 3,003,396, and U.S. Pat. No. 4,327,468).
The prior art variable-crown rolls without loading arms have favorable properties of their own, but they have also involved the drawback that it has become necessary to reduce the dimensions and the rigidity of the stationary central axle of the variable-crown roll, because for the opening movement, a certain intermediate space must be provided between the roll mantle and the central axle in addition to the space that is required at the opposite side relative to the hydraulic loading elements for the deflection of the central axle. Such space requirement is, as a rule, at the maximum at the transverse center plane of the central axle. In view of the structural and functional properties of a variable-crown roll, it is however of extreme importance that the central axle be made as rigid as possible, explicitly in the plane of the nip and in the direction of the hydraulic loading elements. In the opposite direction, the rigidity of the central axle is not of as great importance.